The present invention relates to time division duplex mobile communication systems, more particularly to random access procedures in time division duplex communication systems, and even more particularly to guard periods for use as part of random access procedures in time division duplex communication systems.
In modern cellular radio communication systems, the radio network strictly controls the behavior of the terminal. Uplink transmission parameters like frequency, timing, and power are regulated via downlink control messages from the NodeB (e.g., in systems such as the Universal Mobile Telecommunication Systems—“UMTS”) or other type of base station (BS) to the mobile terminal (e.g., User Equipment—“UE”—in UMTS (3G)-type systems, and Mobile Station—“MS”—in Global System for Mobile communication (GSM)-type systems).
The uplink timing is of particular importance. Modern digital wireless systems make use of time slots or frames. Sharing of the air interface in such systems includes a Time Division Multiple Access (TDMA) component, whereby consecutive time slots and/or frames are allocated to different users. In order to avoid any overlap between consecutive uplink packets from different users, a strict uplink timing control is required.
In cellular networks having a cell radius that can range from a few kilometers to tens of kilometers, it is necessary to take into account the radio signal's time of flight (i.e., the propagation delay between a transmitter antenna and a receiver antenna). That is, the extra time delay over the radio propagation path from the UE results in delayed arrival times of the packets at the NodeB. The amount of additional delay experienced depends on the distance d between the NodeB and the UE. Since the UE timing (in both the uplink and the downlink directions) is based on the downlink control signals, which are by themselves delayed by the same propagation delay, the aggregate timing mismatch, Δt in the uplink amounts to twice the propagation delay d/c, where c is the speed of light. The value Δt thus represents the round-trip delay (RTD) over the air.
In communication systems that use Time Division Duplexing (TDD), the radio transceivers at the NodeB and the UE cannot transmit and receive simultaneously. That is, a transceiver must finalize a complete reception operation before starting transmission, and conversely, an entire transmission operation must be finalized before starting reception. At no point in time may the uplink and downlink signals overlap at the antenna of the NodeB or at the UE.
During operation, the UE must be synchronized with the NodeB. However, upon power-on or after a long standby or sleep time, the UE is not synchronized in the uplink. Unlike the uplink frequency and power estimate, which the UE can derive from the downlink (control) signals, it is difficult to make a timing estimate for the uplink because the round-trip propagation delay between the NodeB and the UE is unknown. Therefore, before commencing traffic, the UE has to carry out a random access (RA) procedure to the network. Since the uplink timing is not time aligned yet, a large guard period is needed. The guard period needs to be at least as long as the maximum aggregate timing mismatch, Δtmax, which is determined by the most distant users, namely the ones on the cell edges which are at the greatest distance from the NodeB. This results in quite an overhead because the guard period cannot be used for transmissions. For cell sizes up to 15 km, a guard period of at least 100 μs is required. For cell sizes up to 30 km, a guard time of at least 200 μs is required.
Due to the unknown round-trip delay, conventional solutions require a large guard time at the RA window. Although the RA procedure is used infrequently (at power-up and when UL synchronization has been lost), this overhead needs to be included in every frame in order to support the most distant users while retaining the latency requirements. For larger cell sizes, longer guard times are required as well as longer preambles in order to preserve the power received by the NodeB. This further increases the overhead.
There is therefore a need for improved RA procedures that do not require such a large resource overhead.